With the widespread popularity of the Internet and personal computers, communication systems with low installation cost and low user fees have been required. As part of the recent establishment of new communication systems, digital subscriber lines and various modifications thereof (e.g., xDSL) that employ twisted pair copper wires that already exist in the telephone lines of every office and every home have become popular.
The term “xDSL” generally refers to numerous types of communication methods that utilize public telephone lines, and include high data-rate DSL (HDSL) for replacing T1 or E1 with twisted-pair copper line, symmetric DSL (SDSL) for transmitting a large amount of data in a Public Switched Telephone Network (PSTN), and so forth.
In another type of DSL system, namely the Asymmetric Digital Subscriber Line (ADSL), the term “Asymmetric” arises from the conditional background that downstream data transmitted from a central office CO to a remote terminal is wider in bandwidth and larger in size when compared to upstream data transmitted from the remote terminal RT to the central office CO. In this case, the downstream data is transmitted from the central office CO to the remote terminal RT. Signal transforming technology used in ADSL includes Carrierless Amplitude Phase modulation (CAP) type systems and Discrete Multi-Tone (DMT) type systems. The DMT type was commonly used by Bell Core of U.S.A. at first in 1993 and adopted as a standard of digital transmission at American National Standards Institute (ANSI), European Telecommunications Standards Institute (ETSI), and International Telecommunications Union (ITU). The DMT type is a method for transferring in parallel by a plurality of narrow band carrier waves. Therefore, the DMT type offers superior noise depression function and inferior interference with other communication lines, as compared to CAP. In addition, ADSL makes high rate data communication available through existing telephone service, and users utilize data communication together with plain old telephone service (POTS). The transmit rate of ADSL is up to 8Mbps for downstream data and 640kbps for upstream data.
The ADSL offers the advantage of using the existing telephone line, while suffering from the disadvantage of being affected by an inferior line environment. Thus, because of the existence of the inferior line environments such as ISDN, HDSL crosstalk and DSL crosstalk, ADSL standards regulate the minimum performance with respect each discrete component at each distance by defining the inferior line environments.
The ADSL is an asymmetric system in which upstream data and downstream data use different frequency bands and interfere with each other. Thus, system performance may be reduced. For example, near-end crosstalk (NEXT) and far-end crosstalk (FEXT) are main causes of reducing system performance of ADSL. The NEXT means that self-transmitting signal affects the receiving terminal, and the FEXT means that downstream data of another central office within an identical bundle affects the receiving terminal. The ADSL system utilizes a telephone line as the data communication medium and includes processes for transmitting and receiving analog signals. The processes for transmitting and receiving are performed in an identical telephone line and utilize an analog filter. However, discrete components consisting of the analog filter for dividing or integrating transmitted and receipt signals have permissible tolerance with respect to given values, such that each analog filter has different characteristics. Different analog filter characteristics mean variation the reflection signals generated by a transmission signal at a receiving terminal. This affects automatic receipt gain control, thereby preventing the system from having optimized performance under given conditions, or inducing system instability.
Conventional automatic receipt gain control determines a maximum value of the received signals and controls a programmable gain amplifier, a scaler, and so on, in order to cause the magnitude of the signal to have a maximum value under given conditions. However, in the case where the characteristic of the analog filter is different from what is used when the maximum value of the receipt signals is determined, the magnitude of the reflected signals, which are generated by the transmission signal at receiving terminal, varies to distort the maximum value of the received signals. That is, the maximum value of the received signals may be judged as being identical to the maximum value defined by the automatic gain control, even though the maximum value of the receipt signals is actually lower. In this case, the system performance is not optimized under the given condition, and is therefore reduced.